In recent years, in order to achieve high reverse breakdown voltage and low loss in semiconductor devices as well as utilization thereof in a high temperature environment, silicon carbide (SiC) has begun to be adopted as a material for the semiconductor devices. Silicon carbide is a wide band gap semiconductor having a larger band gap than that of silicon (Si), which has been widely used as a material for semiconductor devices conventionally. Hence, the adoption of silicon carbide as the material for semiconductor devices will achieve high reverse breakdown voltage, reduced on resistance, and the like in the semiconductor devices. Further, when such semiconductor devices formed using silicon carbide as their material are utilized in a high temperature environment, the semiconductor devices are less deteriorated in properties as compared with semiconductor devices formed using silicon as their material, advantageously.
However, properties required in semiconductor devices are high in recent years. For example, even when SiC is employed as a material of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), further reduction of on resistance therein is still required. For the reduction of on resistance, it is effective to improve channel mobility.
Proposed is a MOSFET structure employing an accumulation mode as an operation mode in the MOSFET. This allegedly improves the channel mobility, thereby reducing on resistance (for example, see Japanese Patent Laying-Open No. 10-308510 (Patent Document 1) and Japanese Patent Laying-Open No. 11-330464 (Patent Document 2)).